Make sure Date and PublishDate is always set to a value if one is available

Fixes #3854

hugolib/page.go

@@ -1147,6 +1147,14 @@ func (p *Page) update(f interface{}) error {
 	}
 	p.Params["draft"] = p.Draft
 
+	if p.Date.IsZero() {
+		p.Date = p.PublishDate
+	}
+
+	if p.PublishDate.IsZero() {
+		p.PublishDate = p.Date
+	}
+
 	if p.Date.IsZero() && p.s.Cfg.GetBool("useModTimeAsFallback") {
 		fi, err := p.s.Fs.Source.Stat(filepath.Join(p.s.PathSpec.AbsPathify(p.s.Cfg.GetString("contentDir")), p.File.Path()))
 		if err == nil {